1. Field of the Invention
This invention relates to the field of automatic transmissions for automotive vehicles. More particularly, the invention pertains to the field of electronic control of hydraulic pressure supplied to an automatic transmission clutch or brake.
2. Description of the Prior Art
Changes among the operating ranges of an automatic transmission are initiated by moving a gear selector lever among the park, reverse, neutral, and forward range positions. Generally, these range selections are made with the engine running at idle speed. When the forward drive range and reverse range are selected, elements of planetary gearsets are interconnected and/or are held against rotation on the transmission casing by engaging hydraulic friction clutches and brakes.
Engagement of these hydraulically-actuated friction elements is made in two phases. During the first phase, called the blip phase, a large magnitude of pressurized hydraulic fluid is applied to the hydraulic cylinder to force a piston against sets of friction discs and into contact with load blocks that react the hydraulic force applied to the piston. During this phase, clearance among these components of the friction element are taken up, but there is little or no torque transmitted through the friction element.
At the end of the blip phase, hydraulic pressure applied to the friction element decreases rapidly to a magnitude substantially below the magnitude of pressure applied during the blip phase. Then, when pressure at this relatively low magnitude is present within the hydraulic cylinder of the friction element, a ramp phase begins and pressure within the cylinder increases linearly over time to a magnitude that produces the maximum torque capacity of the friction element. Engagement of the friction element that produces a change in operating range of the transmission should be completed quickly, i.e., soon after the vehicle operator changes the position of the shift lever, smoothly, as imperceptibly as possible, and without harshness.
Conventionally, the magnitude of pressure during the blip phase and the rate of increase of pressure during the ramp phase are controlled electronically on the basis of the engine speed at the beginning of friction element engagement. Unfortunately, clutch and brake engagements produced by this strategy are inconsistent with respect to the length of time required to complete each engagement and the extent to which the operator can sense their occurrence.